Of all the synthetic polymers considered as materials with useful gas permeation properties, such as resistance to passage of oxygen, carbon dioxide, water, and the like, poly(vinyl alcohol) (PVOH), a polymer made up of units of the structure ##STR1## and generally prepared by the total hydrolysis of homopolymers of vinyl acetate and related vinyl esters, the starting polymer made up of units of the structure ##STR2## where R is H or --(CH.sub.2).sub.m --CH.sub.3 and m is 0 to 7, preferably 0, ranks as the most impervious to the passage of small molecules. PVOH derives this property from the high cohesive energy density and polarity of the hydroxy groups. The presence of the network of hydroxy groups has the concomitant effect of rendering the polymer (PVOH) impermeable to gases, but sensitive to moisture. The strong intermolecular interaction resulting from the high polarity of the --OH functional group gives rise to a melting temperature in the vicinity of the degradation temperature of PVOH. Consequently, melting is accompanied by degradation. The degradation is so severe that PVOH by itself cannot either be melt extruded or injection molded.
The above limitations were surmounted by the preparation and subsequent hydrolysis of vinyl acetate copolymers with monomers other than vinyl esters, especially copolymers with olefins, such as ethylene, propylene, butene-1, and the like. Hydrolysis of ethylene/vinyl acetate copolymers provides a polymer which exhibits those desirable characteristics of PVOH, but is superior to PVOH in performance in hydrophilic environments, such as wet strength, and in melt processability. However, these copolymers exhibit a significant increase in the permeability of the polymer to small molecules. Polymers having a low mol percentage of ethylene, such as from about 5 to about 25 mol percent, are similar to poly(vinyl alcohol) in that they cannot be melt-processed into film without the aid of plasticizers.
In order to render PVOH melt processable, steps have been taken to break up the crystallinity by the addition of external plasticizers. Amongst the best known plasticizers of PVOH are the polyols; these include polyethylene glycol, glycerol, and neopentyl glycol. The use of small molecules or oligomers as plasticizers for PVOH has its inherent limitations and disadvantages. The current state of the art technology employs 10-25 parts of plasticizer to 100 parts of PVOH. A higher concentration of plasticizer leads to phase separation and embrittlement of the plasticized matrix. Low levels of plasticizer, on the other hand, lead to the formation of highly viscous inextrudable melts during melt processing and extrusion. Another shortcoming of plasticized PVOH is the occurrence of plasticizer migration, which arises during thermal processing such as extrusion and heat sealing of PVOH film. During extrusion, the low molecular weight plasticizer may deposit at the die lips. During heat sealing, the low molecular weight plasticizer will migrate and evaporate from the heated region of the film. In the absence of the plasticizer, the PVOH rapidly recrystallizes and embrittles the heat sealed portion of the film. In a packaging application, this embrittlement can compromise the integrity of the package via cracks and pinholes. Another shortcoming of externally plasticized PVOH, which manifests itself when the plasticized PVOH resin comes into contact with alkaline or acidic solvents, is the hydrolysis and subsequent embrittlement of the partially hydrolyzed PVOH resin that is frequently used in preparing plasticized PVOH material.
Preparation of internally plasticized PVOH resin by polymerization of vinyl acetate in the presence of a plasticizer or second polymer has been studied to overcome the above difficulties, but such polymerizations, especially in emulsion, offer limitations caused by the difficulty of dispersing the plasticizer or pre-formed second polymer where it is intimately admixed with the polymerizing vinyl ester, which has a significant degree of water solubility.
In spite of the fact that all of the above mentioned techniques have the effect of improving the melt processing characteristics of PVOH, they also have the concomitant effects of significantly increasing the permeability of the resin to small molecules and reducing the stiffness and heat distortion temperature of the resin. Thus there exists a need for a means to allow melt-processing of polymers of high vinyl alcohol content, such as fully hydrolyzed or highly hydrolyzed polymers of vinyl esters, into useful objects maintaining most of the barrier properties of the polymer of high vinyl alcohol content. There further exists a need for additive polymers which may be blended with polymers of high vinyl alcohol content to enhance their ability to form films and coatings with improved properties of the film or coating without much loss in barrier properties.
In U.S. Pat. No. 5,147,930 are disclosed, among many others, blends of (a) copolymers of poly(C.sub.1 -C.sub.4) alkyl methacrylates with N-vinylpyrrolidone, it being taught that such copolymers could optionally contain (meth)acrylic acid, with (b) polymers and copolymers containing at least 50 mol percent of vinyl alcohol mers (equivalent to the present description of polymers containing units of the structure ##STR3## the amounts of (a) and (b) in the blend being 20 to 95% by weight and 2.5 to about 40% by weight respectively. These specific blends of this Patent are directed towards improved barrier properties of the acrylic polymer, thus the high ratio of (a) to (b).